#include<iostream>
#include<vector>
#include<queue>
#include<cstring>
#include<cmath>
#include<map>
#include<set>
#include<cstdio>
#include<algorithm>
#define debug(a) cout<<#a<<"="<<a<<endl;
using namespace std;
const int maxn=3e5+100;
typedef long long LL;
inline LL read(){LL x=0,f=1;char ch=getchar();	while (!isdigit(ch)){if (ch=='-') f=-1;ch=getchar();}while (isdigit(ch)){x=x*10+ch-48;ch=getchar();}
return x*f;}
LL a[maxn],b[maxn];
bool check(LL x,LL n,LL m){
    for(LL i=1;i<=n;i++) b[i]=a[i];
    for(LL i=1;i<=n;i++){
        if(b[i-1]>b[i]){
            if(b[i-1]-b[i]>x) return false;
            else b[i]=b[i-1];
        }
        else if(b[i]>b[i-1]){
            if(b[i-1]+m-b[i]<=x) b[i]=b[i-1];
        }
    }
    return true;
}
int main(void)
{
  cin.tie(0);std::ios::sync_with_stdio(false);
  LL n,m;cin>>n>>m;
  for(LL i=1;i<=n;i++){
    cin>>a[i];
  }
  LL l=0;LL r=1e17;
  while(l<r){
    LL mid=(l+r)>>1;
    if(check(mid,n,m)) r=mid;
    else l=mid+1;
  }
  cout<<l<<"\n";
return 0;
}
